“o-tagai ni・〜au”: Each Other / Mutually [JLPT N3]
By Nihongo to Japan · Updated July 3, 2026
Expressing that both sides do something to each other
“o-tagai ni・〜au”: Each Other / Mutually (both sides reciprocate)
「お互いに」 is an adverb meaning “each other, mutually”: 「お互いに協力する」 (cooperate with each other). And 「verb ます-stem ＋ 合う」 builds “mutually” right into the verb: 助け合う (help each other), 励まし合う (encourage each other). The two often combine, stressing that both sides do something to each other, equally.
🧠 Core nuance: the action goes both ways
The core is reciprocity: not one-sided A-to-B, but A and B each doing the same to the other. お互いに spells out “each other,” while 〜合う has “mutual” built in (合 = come together / mutually). So 助け合う is “you help me, I help you” — full of a warm, equal, shared feeling.
📌 How to Connect
| Form | Pattern | Example |
|---|---|---|
| お互いに | adverb, modifies a verb | お互いに理解する |
| verb ＋ 合う | ます-stem ＋ 合う | 助け合う／話し合う |
| Idiom | お互い様 (おたがいさま) | 困ったときはお互い様 |
💬 Example Sentences
- チームでは、お互いに協力することが大事だ。(In a team, cooperating with each other matters.) — adverb
- 困ったときは助け合うのが家族だ。(Helping each other in hard times is what family is.) — 〜合う
- 二人は本音を話し合った。(The two talked things out honestly with each other.) — mutual
- 「ありがとう」「いいえ、お互い様ですよ」。(“Thanks.” “No, no — we're even / same to you.”) — idiom
- 選手たちは試合後、健闘を讃え合った。(After the match, the players praised each other's effort.)
🔄 Compare: お互いに/〜合う vs 一緒に vs 互いに vs し合う
| Expression | Nuance | Example |
|---|---|---|
| お互いに | each other, mutually (two-way, equal) | お互いに助ける |
| 〜合う | “mutual” built into the verb | 助け合う |
| 一緒に | “together” (jointly, not necessarily mutual) | 一緒に行く |
| 互いに (たがいに) | same as お互いに, more written | 互いに尊重する |
⚠️ Common Mistakes
- 〜合う with the dict. form: ✗ 助けるあう → ✓ 助け合う (ます-stem ＋ 合う).
- 〜合う for one-sided actions: 〜合う must be two-way (mutual); not for one-sided actions.
- Misusing お互い様: it's a polite “same to you / we're even,” used to respond to thanks/apology — not a general “the same.”
- Confusing it with 一緒に: mutual action uses お互いに/〜合う; doing together uses 一緒に.
💡 Nuance & When to Use
お互いに・〜合う is the core way to express mutuality — for team cooperation, helping each other, mutual understanding, full of equal, warm humanity. お互いに (adverb) spells it out, 〜合う bakes “mutual” into the verb, and they often pair up. お互い様 is also a super-handy everyday courtesy. Master it to naturally express two-way relationships. A must at N3.
🎯 JLPT Exam Tips
- High-frequency N3: お互いに (adverb); verb ます-stem ＋ 合う.
- Connection: 〜合う takes the ます-stem (助け合う), not the dictionary form.
- Distinction: お互いに/〜合う (mutual) vs 一緒に (joint).
🖊️ 練習題（5題）
Q1. 「チームでは、___協力することが大事だ。」（互相協力）
(A) お互いに (B) わざと (C) せっかく (D) もともと
Q2. 「助ける」を「互相幫助」にすると?
(A) 助け合う (B) 助けるあう (C) 助けてあう (D) 助かり合う
Q3. 「お互い様」の意味は?
(A) 彼此彼此（互相一樣）
(B) 只有我
(C) 只有你
(D) 都不是
Q4. 「〜合う」の前に来る動詞の形は?
(A) ます形語幹
(B) 辭書形
(C) た形
(D) て形
Q5. 接法が正しいのは?
(A) 励まし合う。 (B) 励ます合う。
(C) 励まして合う。 (D) 励ました合う。
答案解析
1. (A) お互いに ── 互相協力。
2. (A) 助け合う ── ます形語幹＋合う。
3. (A) ── 彼此彼此。
4. (A) ます形語幹 ── 〜合う 接ます形語幹。
5. (A) ── 励まし(ます形語幹)＋合う。